Macroalgae filling the habitat void following catastrophic losses of seagrass in the Indian River Lagoon, FL

The Indian River Lagoon (IRL) on Florida’s east-central coast is a highly eutrophic, urbanized estuary where, beginning in 2011, multiple harmful phytoplankton blooms were followed by catastrophic seagrass losses. Since then, in many locations where seagrass was lost, the rhizophytic green macroalga Caulerpa prolifera has become the dominant benthic cover. Although the habitat value of C. prolifera compared to the seagrass Halodule wrightii was assessed in the IRL during the late 1980s, there is no information regarding its current habitat value following the catastrophic losses of seagrass. Therefore, the habitat function of C. prolifera in the IRL was assessed during a period of very low seagrass cover by quantitively sampling epifauna inhabiting this macroalga. The benthic habitat cover and faunal composition of four C. prolifera sites in the IRL were determined between 2020–2021. Benthic cover varied by site and event with variable % cover of C. prolifera overall. The faunal composition of C. prolifera was similar to what was previously observed for H. wrightii in the IRL, however faunal densities were significantly lower than historic estimates, which is critical information for resource managers. Incidentally sampled red drift macroalgae supported epifaunal species similar to C. prolifera also demonstrating its current habitat value in the IRL. Thus, these macroalgal habitats may be serving as refugia for estuarine fauna in the relative absence of seagrasses, however faunal densities may have declined from historic levels. These findings may be useful to other locations experiencing catastrophic seagrass losses combined with large-scale macroalgal blooms.